Sensor protection circuit

ABSTRACT

The invention relates to a sensor protection circuit for at least one sensor ( 6 ), especially in a motor vehicle electric system, comprising at least one supply line ( 3, 4 ) for supplying current to the sensor ( 6 ) and a current measuring unit ( 8 ) for detecting the electric current flowing through the supply line ( 3 ), in order to prevent damage to the sensor ( 6 ) as a result of excess voltage. Said current measuring unit ( 8 ) is connected to a current limiting device or to a switch element in order to limit the current or to disconnect the supply line ( 3 ).

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a sensor protection circuit, in particular in amotor vehicle electric system, having at least one supply line forsupplying current to the sensor.

Numerous sensors are typically used in modern motor vehicles to detectthe state of the motor vehicle or the environment. Power can be suppliedto the sensors by means of separate supply lines which are not part ofthe motor vehicle electric system. This is advantageous in that thesensor interrogation is, as a rule, not interrupted by the significantlyhigher load currents of the motor vehicle electric system.

A network voltage of 12 V has hitherto been the norm in motor vehicleelectric systems, whereas power is supplied to the sensors at a lowervoltage of 5 V for example. If there is a short circuit between a supplyline of a sensor and the voltage conducting line of the motor vehicleelectric system, the higher electric system voltage of 12 V is appliedto the sensors, which could result in damage to the sensors. It istherefore known to provide the sensors with a passive protection circuitwhich also prevents damage to the sensors caused by a short circuitbetween a supply line for the sensors and the motor vehicle electricsystem, so that the sensors are short-circuit stable up to the motorvehicle electric system voltage of 12 V. This type of protective circuitfor the sensors can comprise Zener diodes or capacitors for example.

Motor vehicle electric systems with a voltage of 42 V are however underdevelopment as a result of the increasing energy requirement ofelectrical motor vehicle components. As a rule the short-circuitstability of the known sensors is however not sufficient to withstand ashort circuit in respect of a voltage of 42 V, so that known sensorswould be damaged in the event of a short circuit if used in a modernmotor vehicle electric system with a voltage of 42 V.

One possible solution to this problem lies in developing new sensorswith a correspondingly high short-circuit stability, which however wouldinvolve considerable development costs.

SUMMARY OF THE INVENTION

The object of the invention is thus to enable the use of known sensorswith a short-circuit stability up to a voltage of 12 V in a modern motorvehicle electric system with a voltage of 42 V, without risking damageto the sensors in the event of a short circuit.

The object is achieved by the features of Claim 1.

The invention comprises the general technical disclosure of applying asensor protection system which detects the current increase on thesupply line with a short circuit of a supply line for a sensor, therebyenabling the introduction of suitable countermeasures.

If a fault occurs, the excess voltage protection for the sensor firstcomes into effect. Fast fault recognition and a similarly rapidintroduction of countermeasures allow the use of sensors with ashort-circuit stability lying below the nominal voltage of the electricsystem.

This advantageously enables the use of conventional sensors with a shortcircuit stability of 12 V in future motor vehicle electric systems witha network voltage of 42 V, without costly new developments of thesensors being necessary.

In the simplest case, the countermeasures employed in the event of ashort circuit can consist of the supply line for the sensor beingdisconnected by a circuit element, thereby preventing a further currentincrease. The disconnection of the supply line for the sensors canoptionally take place at one pole for the ground line or the voltageline or at two poles for the ground line and the voltage line, whereby acircuit element is preferably used which is switched in sequence with asupply line.

In this version of the invention, the circuit element can be activatedexternally in order to disconnect the supply line for the sensorsindependently from the current on the supply line, the sensor protectioncircuit preferably comprising a separate control input. This separatedisconnection can be software-controlled for example, if a short circuitlasts for a predetermined time.

As a countermeasure in the event of a short circuit of a supply line fora sensor, it is preferable that the supply line is not completelydisconnected but that a current limitation on the supply line for thesensor is instead implemented in order to prevent damage to the sensor.

The current on the supply line for the sensors is preferably measured onthe ground line, however it is essentially also possible to measure thecurrent on the voltage line of the sensors.

Furthermore the sensor protection circuit according to the inventionpreferably also comprises a passive protective circuit in order toprevent damage to the sensors in the event of a short circuit.

A protective circuit of this type can comprise capacitors or Zenerdiodes for example, whereby these components can be switched between thetwo supply lines for the sensors.

However, it is also possible for the components of the passiveprotective circuit to connect the ground line and/or the voltage line ofthe sensors to ground respectively.

The use of Zener diodes as passive protective circuits is advantageousif the Zener diodes in the sensor protection circuit according to theinvention have a lower breakdown voltage than the Zener diodes which aretypically provided in the sensors as protective circuits. This isadvantageous since after a short build-up phase, the short circuitcurrent flows exclusively through the Zener diodes in the sensorprotection circuit and no longer through the Zener diodes in thesensors, so that the Zener diodes in the sensors only have to carry theshort circuit current for a very short period of time.

The term sensor used within the context of the invention is generallyunderstood to comprise all components which are electrically suppliedand provide a measurable variable. By way of example, Lambda sensors,temperature sensors, pressure sensors, inclination sensors andacceleration sensors are worth mentioning.

Nor is the invention limited to use in future motor vehicle electricsystems having a network voltage of 42 V. Instead it is also possible asa result of a corresponding standardization that electric circuits withother network voltages will be developed in which the sensor protectioncircuit according to the invention will also be able to be used.

Other advantageous embodiments of the invention are set down in thesubclaims and are described below together with the description of thepreferred exemplary embodiment of the invention with reference to thedrawings, in which;

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sensor protection circuit according to the invention witha sensor in the form of a circuit diagram, and

FIG. 2 shows a current limitation circuit of the sensor protectioncircuit in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The circuit diagram displayed in FIG. 1 shows a sensor protectioncircuit 1 according to the invention which is used in a motor vehicleelectric system with a network voltage of 42 V, to enable a conventionalsensor unit 2 with a short circuit stability of 12 V to be operated inthe event of a short circuit without damaging the sensor unit 2 orcompletely destroying it.

For clarification purposes, FIG. 1 only shows the sensor unit 2, howeverseveral sensor units can be operated on the sensor protection circuit 1,as indicated by the dashed lines.

For supplying current to the sensor unit 2, the sensor protectioncircuit 1 comprises a ground line 3 and a voltage line 4, whereby thevoltage line 4 is connected on one side to a voltage regulator 5, whichadjusts the voltage on the voltage line 4 to the operating voltage ofthe sensor unit 2, amounting to 5 V in this exemplary embodiment. Incontrast, the ground line 3 is connected to ground, with a buffercapacitor C1 arranged at the input of the sensor protection circuit 1between the voltage line 4 and the ground line 3.

The sensor unit 2 comprises the actual sensor 6, which measures thephysical variable quantity, such as temperature, pressure, or air-fuelratio λ and outputs a corresponding measurement signal via a signal line7.

Furthermore, the sensor unit 2 comprises a passive protective circuit inorder to prevent damage to the sensor 6, in the case of a short circuit,with the passive protective circuit of the sensor unit 2 being designedfor the hitherto conventional electric system voltage of 12 V.

The passive protective circuit of the sensor unit 2 comprises a Zenerdiode D1 on the one hand, having a breakdown voltage of U_(Z1)=16V,whereby, in the event of an excess voltage, the Zener diode D1 preventsan input-side overload of the sensor 6. The Zener diode D1 is thusswitched between the signal line 7 and the ground line 3.

Furthermore, the passive protective circuit of the sensor unit 2comprises a further Zener diode D2 having a breakdown voltage ofU_(Z2)=16V, which prevents an input-side excess voltage of the sensor 6in the event of an excess voltage. The Zener diode D2 is thus switchedbetween the voltage line 4 and the ground line 3.

The passive protective circuit of the sensor unit 2 only offers shortcircuit stability in relation to the hitherto conventional electricsystem current of 12 V, whereas the passive protective circuit of sensorunit 2 is overstrained if used in a modern motor vehicle electric systemwith a voltage of 42 V.

The sensor protection circuit 1 therefore comprises a current limitingdevice 8, which is arranged in the ground line 3 for the sensor unit 2and measures the current through the ground line 3, with the currentlimiting device 8 being shown in detail in FIG. 2 and described below.

The current limiting device 8 has a measurement resistor R1 for which isarranged in the ground line 3, so that the voltage drop through themeasurement resistor R1 reflects the current which flows through theground line 3.

Furthermore, a MOSFET transistor T1 is arranged in the ground line 3,which, as a function of its activation, enables both a currentlimitation as well as a complete disconnection of the ground line 3 inthe event of a short circuit.

The gate connection of the MOSFET transistor is connected to the supplycurrent U_(v)=+5V via a resistor R2, so that the MOSFET transistor T1through connects during normal operation.

Furthermore, the gate connection of the MOSFET transistor T1 isconnected to the output of a comparator circuit comprising two resistorsR3, R4 and two transistors T2 and T3. The comparator circuit isconnected on one side to the measurement resistor R1 and thus detectsthe electrical current flowing through the ground line 3. The comparatorcircuit controls the gate connection of the MOSFET transistor T1 as afunction of the electrical current flowing through the ground line 3,such that the current is limited in order to prevent damage to thesensor 6 in the event of a short circuit.

The current limiting device 8 further comprises a transistor T4 whichconnects the gate connection of the MOSFET transistor T1 to ground, sothat the MOSFET transistor T1 disconnects the ground line 3 if thetransistor T4 through connects, since the potential of the gateconnection of the MOSFET transistor T1 is drawn towards ground.Transistor T4 is activated via a separate control unit 9 and tworesistors R5, R6 connected in series by means of a software control,which disconnects the MOSFET transistor T1 if the short circuit lastsfor a predetermined period of time.

Furthermore, the sensor protection circuit 1 used to protect the sensor6 also comprises a passive protective circuit with two Zener diodes D3,D4 and three capacitors C2, C3 and C4.

The Zener diode D3 is thus switched between the signal line 7 and theground line 3 and prevents an output-side overload of the sensor 6 witha short circuit between the electric system voltage of 42 V and thesignal line 7. The breakdown voltage of the Zener diode D3 thus amountshere to U_(Z3)=8V and is thus lower than the Zener voltage U_(Z1)=16V ofthe Zener diode D1. This is advantageous since a possible short circuitcurrent can then only flow through the Zener diode D1 during a shortbuild-up time and is subsequently taken over by the Zener diode D3. Inthis way the Zener diode D1 must only carry a possible short circuitcurrent for a relatively short amount of time, thereby preventing anoverload of the passive protective circuit of the sensor unit 2.

In contrast, the Zener diode D4 is switched between the voltage line 4and the ground line 3, and prevents an input-side overload of the sensor6 in the event of a short circuit between the electric system networkvoltage of 42 V and the voltage line 4. The breakdown voltage of theZener diode D4 here amounts similarly to U_(Z4)=8V and is thus lowerthan the Zener voltage U_(Z2)=16V of the Zener diode D2. This isadvantageous, since a possible short circuit current only flows throughthe Zener diode D2 during a short build-up time, and is subsequentlytaken over by the Zener diode D4. In this manner, the Zener diode D2must only carry a possible short circuit current for a relatively shorttime span, thereby preventing an overload of the passive protectivecircuit of the sensor unit 2.

The capacitor C2 is switched between the signal line 7 and ground andthus buffers EMC spikes on signal line 7, which also counteracts anoutput-side overload of the sensor 6.

In contrast, the capacitor C3 connects the voltage line 4 to the groundline 3, thereby buffering voltage fluctuations at the input of thesensor unit 2 and taking over the current.

Finally, the capacitor C4 connects the ground line 3 to ground, whichattenuates EMC fluctuations of the ground potential on the ground line3.

In the event of a short circuit after a short build-up time, the passiveprotective circuit of the sensor protection circuit 1 takes over theshort circuit current of the passive protective circuit of sensor unit2, thereby preventing an overload of the passive protective circuit ofthe sensor unit 2.

The current limiting device 8 limits the short circuit current to theground line 3, in order to prevent an overload of the passive protectivecircuit of the sensor protection circuit 1. With a continuous shortcircuit the control input 9 of the current limiting device 8 is thenactivated, whereupon the MOSFET transistor T1 completely disconnects theground line 3.

The invention is not restricted to the aforementioned preferredexemplary embodiment. In fact, a number of variants and modificationsare possible which similarly make use of the inventive idea and thusfall into the protected zone.

1. A sensor protection circuit for at least one sensor, the protectioncircuit which comprises: at least one supply line for supplying currentto the sensor; a current measuring unit for detecting an electriccurrent flowing through said supply line for preventing damage to thesensor from excess voltage; said current measuring unit having connectedthereto at least one of a current limiting device for limiting theelectric current and a circuit element for disconnecting said at leastone supply line; and a separate control input connected to said circuitelement, in a case of a short circuit lasting for a predetermined periodof time said separate control input driving said circuit element into anon-conducting state resulting in said at least one supply line beingdisconnected.
 2. The sensor protection circuit according to claim 1,wherein said supply line monitored by the current measuring unit is aground line for the sensor.
 3. The sensor protection circuit accordingto claim 1, which further comprises a signal line for recording a sensorsignal from the sensor.
 4. The sensor protection circuit according toclaim 1, which comprises a voltage line and a ground line each connectedto and supplying current to the sensor.
 5. The sensor protection circuitaccording to claim 4, which comprises means for preventing excessvoltages connected in at least one of said voltage line, said groundline, and a signal line connected to the sensor.
 6. The sensorprotection circuit according to claim 5, wherein said means includes atleast one of a Zener diode and at least one capacitor.
 7. The sensorprotection circuit according to claim 6, wherein said Zener diode and/orsaid at least one capacitor is connected between the voltage line or thesignal line and the ground line.
 8. A sensor protection circuit for atleast one sensor, the sensor protection circuit comprising: at least onesupply line for supplying current to the sensor, said at least onesupply line including a voltage line and a ground line each connected toand supplying the current to the sensor; a current measuring unit fordetecting an electric current flowing through said supply line forpreventing damage to the sensor from excess voltage; said currentmeasuring unit having connected thereto at least one of a currentlimiting device for limiting the electric current and a circuit elementfor disconnecting said at least one supply line; means for preventingexcess voltages connected in at least one of said voltage line, saidground line, and a signal line connected to the sensor, said meanshaving at least one of a Zener diode and at least one capacitor; andsaid sensor containing at least one Zener diode for short circuitprotection, and said Zener diode for preventing excess voltages having alower breakdown voltage than said Zener diode of said sensor.
 9. Incombination with a motor vehicle, a sensor protection circuit for asensor connected in an electric circuit of the motor vehicle, the sensorprotection circuit according to claim 1.